This invention relates to a single panel color projection liquid crystal display (called, a single panel color projection LCD hereinafter) to project onto a screen a color image given by a color liquid crystal panel and to thereby display an enlarged color picture on the screen. In particular, this invention relates to a single panel color projection LCD which is for use in combination with a personal computer, a TV set, a VCR, and so on.
Color projection LCDs can be classified into two types, a single panel color projection LCD having one liquid crystal panel and a triple panel color projection LCD having three liquid crystal panels.
Roughly speaking, the triple panel color projection LCD displays an image as follows. Light emitted from a white light source is separated into three primary color beams, namely, red, green, and blue beams or rays by the use of dichroic mirrors. The three color beams are irradiated onto three liquid crystal panels corresponding to the colors of the color beams. Then, the light beams passing through the panels are optically compounded and projected onto a screen. Thus, the LCD displays an enlarged picture of the image from optical input devices, such as a personal computer and a TV set. Though the triple panel color projection LCD has a good optical efficiency and produces higher image resolution, it is more complex and larger in structure of the optical system and more expensive than the single panel color projection LCD.
On the other hand, the single panel color projection LCD is more advantageous than the triple panel color projection LCD with regard to size and cost performance.
Taking this into consideration, description will be made only about the single panel color projection LCD. Specifically, such an LCD basically comprises a liquid crystal panel having color filters corresponding to each of the three color beams and serves to display a color image in a liquid crystal panel and to project an enlarged color image on a screen. Such an LCD is advantageous in compactness and inexpensiveness.
However, this type of LCD has a following problem. In the liquid crystal panel, the color filters absorb about two-thirds of each light beams irradiated to the panel, and therefore, the single panel color projection LCD can effectively utilize only about one-third of the irradiated light beams to project an image.
A conventional technique to solve the above-mentioned problem is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 60538/92. The conventional LCD comprises three dichroic mirrors and a liquid crystal panel. In detail, a predetermined arrangement of the dichroic mirrors makes it possible to separate light from a white light source into the three primary color beams--blue, red, and green beams, and irradiate the primary color separated beams onto the liquid crystal panel. The liquid crystal panel including a microlens array, converges each primary color beam irradiated by each dichroic mirror into different pixels of the liquid crystal panel. Each color beam passing through each pixel is optically compounded, and forms a color image on a screen.
A conventional LCD of the type described can project a picture of a color image onto a screen by the use of a combination of the three dichroic mirrors and the liquid crystal panel. In this case, since the liquid crystal panel requires no color filter, the absence of the optical power loss caused by the color filter enables the efficient use of light from a light source.
However, the conventional LCD still has a difficulty to display a bright image.
In case of the above-mentioned LCD, the three dichroic mirrors separate light from a light source into the three primary color beams--blue, green, and red beams, by selecting particular wavelengths corresponding to the three primary colors. Though the three primary color beams irradiated from the three dichroic mirrors are substantially natural light, it is necessary that three primary color beams irradiated to the liquid crystal panel are linearly polarized light. Therefore, the color separate beams irradiated from the dichroic mirrors to the liquid crystal panel must be polarized by a polarizer positioned between the three dichroic mirrors and the crystal panel.
However, only a small portion of the color separated beams is polarized, while most amount of the color beams remains natural light.
Generally, when natural light is irradiated, a polarizer selects only linearly polarized light by passing either P-polarized or S-polarized light through and by reflecting or absorbing the unselected polarized light.
Consequently, the light irradiated on the liquid crystal panel has approximately a half of the optical power, compared with the light emitted by a light source. This is why the polarizer causes the loss of the optical power by absorbing or reflecting either of two kinds of the polarized components included in the light emitted by the light source. In case of this conventional single panel LCD, the loss caused by the polarizer is unavoidable with respect to its structure. As a result, it is difficult for the conventional LCD to display a bright color image.
Moreover, consideration may be made about using a high power light source to compensate for the optical power loss. However, such compensation for the optical power loss by the use of the high power light source leads to a following problem. As is known well in the art, heat is generated when a polarizer absorbs light. The heat generated by the absorption of light heats the polarizer. In case of a large quantity of heat, it lowers the polarizer's performance. Therefore, it is preferable that the polarizer absorbs as small optical power as possible. From this point of view, the conventional LCD cannot employ a high power light source to make up for the optical power loss. As a result, the conventional LCD is difficult to display a bright color image.